1. Field of the Invention
The present invention relates to an image reading apparatus. More particularly, the present invention relates to a line image scanner for optically reading a document sheet in full color. The present invention also relates to an image sensor chip advantageously used for such an image scanner.
2. Description of the Related Art
An example of conventional image reading apparatus is shown in FIG. 14 of the accompanying drawings. The illustrated reading apparatus includes a light source unit Ba made up of a cold-cathode tube 1xe2x80x2 and a mirror 2xe2x80x2. The conventional apparatus also includes an inverter unit Bb for providing driving power to the cold-cathode tube 1xe2x80x2 via a flexible cable 3xe2x80x2, and a light leading unit Bc provided with mirrors 4axe2x80x2, 4bxe2x80x2. The conventional apparatus further includes an image reading unit Bd provided with a lens 5xe2x80x2 and an image sensor 6xe2x80x2.
In operation, the light source unit Ba is reciprocated in the secondary scanning direction under a stationary glass plate 7xe2x80x2 on which a document sheet K to be read out is placed. Accordingly, the light leading unit Bc is repeatedly moved in the secondary scanning direction.
The conventional image reading apparatus has been found to be disadvantageous in the following points.
First, the light source unit Ba, the inverter unit Bb and the light leading unit Bc are produced separately from each other. With such an arrangement, it is difficult to accurately position these units to each other. Further, production costs tend to be high since a plurality of separate units need to be manufactured.
Still further, the flexible cable 3xe2x80x2 connecting the light source unit Ba to the inverter unit Bb is long enough, so that the reciprocating movement of the light source unit Ba is not hindered. However, as the length of the flexible cable 3xe2x80x2 increases, the loss of the driving power provided by the inverter unit Bb to the cold-cathode tube 1xe2x80x2 increases. As a result, the luminance of the cold-cathode tube 17 may be unduly reduced.
The conventional image reading apparatus also has the following disadvantage.
Though not shown in FIG. 14, the image sensor 6xe2x80x2 includes a plurality of image sensor chips. Each image sensor chip is formed with three rows of light receiving elements extending in the primary scanning direction. A first row is made up of red light receiving elements used for selectively detecting a red component of white light. Similarly, a second row is made up of green light receiving elements used for selectively detecting a green component of white light, while a third row is made up of blue light receiving elements used for selectively detecting a blue component of white light.
Each row of light receiving elements has a pitch P between the light receiving elements in the primary scanning direction. The pitch between the light receiving elements in the secondary scanning direction (which is perpendicular to the primary scanning direction) is also P. As viewed in the secondary scanning direction, each of the light receiving elements has a length of P/2.
For providing color selectivity, color filters are used for the light receiving elements. Specifically, each red light receiving element is covered by a red filter which allows selective permeation of red light, whereas each green light receiving element is covered by a green filter which allows selective permeation of green light. Similarly, each blue light receiving element is covered by a blue filter which allows selective permeation of blue light.
In the conventional image reading apparatus, no attention has been paid to e.g., the thickness of the color filters, and three types of color filters may have the same thickness. With such an arrangement, however, it may be impossible to realize a high-fidelity reproduction of the read image. This is partly because properties of the color filters are not the same for the different color lights (red, green, blue) and partly because properties of the light receiving elements are not the same for the different color lights, either.
Specifically, as shown in FIG. 15, among the three colors (Red, Green and Blue), a conventionally available light receiving element (e.g., phototransistor) has the highest relative sensitivity for red, the second highest relative sensitivity for green, and the lowest relative sensitivity for blue.
FIG. 16 shows relationship between the wave length of incident light and the transmittance of the respective color filters (Red filter, Green filter and Blue filter). As illustrated, the red filter has the highest transmittance, while the green filter has the second highest transmittance, and the blue filter has the lowest transmittance.
As shown in FIG. 17, a typical cold-cathode tube generates white light whose green component has the highest energy ratio compared with those of the red and blue components.
FIG. 18 shows the reflectivity of red (R), green (G) and blue (B) lights on four types of test charts (WHITE, RED, GREEN, BLUE and BLACK test charts). As illustrated, of three colors, blue light is reflected most on the white test chart. Red light is reflected most on the red test chart, green light is reflected most on the green test chart, and blue light is reflected most on the blue test chart. When the test chart is black, the three color lights are hardly reflected.
FIG. 19 is obtained from a combination of FIG. 16 and FIG. 18. As is shown, when using the white test chart, the green light has the highest product of the transmittance and the reflectivity.
FIG. 20 is obtained from a combination of FIG. 15, FIG. 17 and FIG. 19. FIG. 20 shows the output voltages generated by the three types of light receiving elements (Red, Green and Blue) when the four types of test charts (WHITE, RED, GREEN, BLUE and BLACK) are irradiated with white light. As is shown, when using the white test chart, the green light receiving element generates the highest output voltage. When using the red test chart, the red light receiving element generates the highest output voltage.
Still further, the conventional image reading apparatus is disadvantageous in the following point.
Referring to FIG. 21, for performing image reading for one line, the light receiving elements 8xe2x80x2 (only one shown) of each image sensor chip are advanced by the distance P in the secondary scanning direction with respect to the document sheet. During this movement, however, the light receiving element 8xe2x80x2 scans a rectangular area having a length of (P+L). This means that image reading for each line is performed for an unduly larger area due to the length L of the light receiving element itself. As a result, with the use of the conventional image reading apparatus, a high-fidelity printout reproduction of the image carried by the document sheet may not be realized.
It is, therefore, an object of the present invention to provide an image reading apparatus capable of overcoming the disadvantages described above.
Another object of the present invention is to provide an image sensor chip advantageously incorporated in such an image reading apparatus.
According to a first aspect of the present invention, there is provided an image reading apparatus comprising:
a light source for irradiating a document sheet with light;
a power supplier for providing the light source with driving power;
a connection cable for electrically connecting the light source to the power supplier;
at least one row of light receiving elements arranged in a primary scanning direction for detecting the light reflected on the document sheet;
a lens array for focusing the reflected light at the row of light receiving elements;
a printed circuit board for mounting the row of light receiving elements thereon; and
a case for supporting the light source, the power supplier, the lens array and the printed circuit board.
The image reading apparatus may further comprise a light reflecting holder formed with a groove for accommodating the light source. The image reading apparatus may also comprise a shield frame for accommodating the light reflecting holder.
Preferably, the shield frame is grounded.
According to a preferred embodiment of the present invention, the case is formed with a first hollow portion for accommodating the light source, a second hollow portion for accommodating the power supplier and a third hollow portion for accommodating the lens array. Further, the case is formed with a cutout for causing the first and the second hollow portions to communicate with each other, the connection cable extending through the cutout.
The light source may comprise a cold-cathode tube, and the power supplier may comprise an inverter.
According to a second aspect of the present invention, there is provided an image reading apparatus comprising:
a light source for irradiating a document sheet with light;
a row of red light receiving elements arranged in a primary scanning direction for detecting a red component of the light reflected on the document sheet, each red light receiving element having a length of L in a secondary scanning direction which is perpendicular to the primary scanning direction;
a row of green light receiving elements arranged in the primary scanning direction for detecting a green component of the reflected light, the row of green light receiving elements being displaced from the row of red light receiving elements by a distance of P in the secondary scanning direction, each green light receiving element having a length of L in the secondary scanning direction;
a row of blue light receiving elements arranged in the primary scanning direction for detecting a blue component of the reflected light, the row of blue light receiving elements being displaced from the row of green light receiving elements by a distance of P in the secondary scanning direction, each blue light receiving element having a length of L in the secondary scanning direction; and
a signal selector;
wherein, in performing image reading for one scanning line, the signal selector is arranged to adopt, as a necessary image signal, a voltage generated by each of the light receiving elements during a period when said each light receiving element is moved in the secondary scanning direction relative to the document sheet by a first feed distance, the signal selector being also arranged to disregard, as an unnecessary image signal, a voltage generated by said each light receiving element during a period when said each light receiving element is moved in the secondary scanning direction relative to the document sheet by a second feed distance subsequent to the first feed distance.
The signal selector may be realized by a CPU, a gate array, or a PLA (programmable logic array).
The first feed distance may be equal to (Pxe2x88x92L), and the second feed distance may be equal to L.
Alternatively, the first feed distance may be smaller than (Pxe2x88x92L), and the second feed distance may be greater than L.
The light source may comprise a cold-cathode tube or a light-emitting diode.
According to a third aspect of the present invention, there is provided an image reading apparatus comprising:
a light source for irradiating a document sheet with light;
a row of red light receiving elements arranged in a primary scanning direction for detecting a red component of the light reflected on the document sheet, each red light receiving element having a length of L in a secondary scanning direction which is perpendicular to the primary scanning direction;
a row of green light receiving elements arranged in the primary scanning direction for detecting a green component of the reflected light, the row of green light receiving elements being displaced from the row of red light receiving elements by a distance of P in the secondary scanning direction, each green light receiving element having a length of L in the secondary scanning direction;
a row of blue light receiving elements arranged in the primary scanning direction for detecting a blue component of the reflected light, the row of blue light receiving elements being displaced from the row of green light receiving elements by a distance of P in the secondary scanning direction, each blue light receiving element having a length of L in the secondary scanning direction;
a light controller;
wherein, in performing image reading for one scanning line, the light controller is arranged to turn on the light source during a period when each of the light receiving elements is moved in the secondary scanning direction relative to the document sheet by a first feed distance, the light controller being also arranged to turn off the light source during a period when said each light receiving element is moved in the secondary scanning direction relative to the document sheet by a second feed distance subsequent to the first feed distance.
The light controller may be realized by a CPU, a gate array or PLA.
The image reading apparatus may further comprise an output timing controller for regulating output of an image signal from each of the light receiving elements, so that the image signal is output from said each light receiving element when the light source is turned off.
According to a fourth aspect of the present invention, there is provided an image sensor chip comprising:
a chip substrate;
a row of red light receiving elements formed in the chip substrate and arranged in a first direction for detecting red light, each of the red light receiving elements being covered by a red filter;
a row of green light receiving elements formed in the chip substrate and arranged in the first direction for detecting green light, each of the green light receiving elements being covered by a green filter; and
a row of blue light receiving elements formed in the chip substrate and arranged in the first direction for detecting blue light, each of the blue light receiving elements being covered by a blue filter;
wherein the red, green and blue filters have predetermined thicknesses, the thickness of the red filter being greater than the thickness of the green filter, the thickness of the green filter being greater than the thickness of the blue filter.
According to a fifth aspect of the present invention, there is provided an image sensor chip comprising:
a chip substrate;
a row of red light receiving elements formed in the chip substrate and arranged in a first direction for detecting red light, each of the red light receiving elements having a red light receiving surface covered by a red filter;
a row of green light receiving elements formed in the chip substrate and arranged in the first direction for detecting green light, each of the green light receiving elements having a green light receiving surface covered by a green filter; and
a row of blue light receiving elements formed in the chip substrate and arranged in the first direction for detecting blue light, each of the blue light receiving elements having a blue light receiving surface covered by a blue filter;
wherein an area of the red light receiving surface is smaller than an area of the green light receiving surface, the area of the green light receiving surface being smaller than an area of the blue light receiving surface.
According to a sixth aspect of the present invention, there is provided an image sensor chip comprising:
a chip substrate;
a row of red light receiving elements formed in the chip substrate and arranged in a first direction for detecting red light, each of the red light receiving elements being covered by a red filter;
a row of green light receiving elements formed in the chip substrate and arranged in the first direction for detecting green light, each of the green light receiving elements being covered by a green filter;
a row of blue light receiving elements formed in the chip substrate and arranged in the first direction for detecting blue light, each of the blue light receiving elements being covered by a blue filter;
a first amplifier for the row of red light receiving elements;
a second amplifier for the row of green light receiving elements; and
a third amplifier for the row of blue light receiving elements;
wherein an amplification factor of the first amplifier is smaller than an amplification factor of the second amplifier, the amplification factor of the second amplifier being smaller than an amplification factor of the third amplifier.
Other features and advantages of the present invention should become clear from the detailed description to be made hereinafter referring to the accompanied drawings.